Over the years, engineers have been challenged to devise a number of different strategies toward the goal of a cleaner burning engine. Experience has taught that various injection timings, quantities and rates have a variety of different desirable results over the complete operating range of a given engine. Therefore, fuel injection systems with a variety of different capabilities can generally outperform fuel injection systems with narrower capability ranges, at least in their ability to reduce undesirable emissions. For instance, the leap from cam control to electronic control in fuel injection systems has permitted substantially lower emissions in several categories, including but not limited to NOx, hydrocarbons and smoke.
One area that appears to show promise in reducing undesirable emissions is often referred to as homogenous charge compression ignition (HCCI). In an HCCI engine, fuel is injected early in the compression cycle to permit thorough mixing with cylinder air to idealy form a lean homogeneously mixed charge before conditions in the cylinder cause auto-ignition. Engines operating in an HCCI mode have shown relatively low outputs of undesirable emissions. Although an HCCI strategy appears promising, it has its own problems. For instance, HCCI can cause extremely high cylinder pressure rise rates and force loads, rendering it most desirable at the lower half of the engine's operating range. Many are also seeking ways to address the difficulty in controlling ignition timing in engines operating with an HCCI strategy. Thus, at this time, a pure HCCI strategy is not viable for most commercial engine applications with conventional power density requirements.
This limitation of HCCI engines has been addressed in the art by equipping an engine with an HCCI fuel injection system and a conventional fuel injection system. Although such a dual system strategy appears viable, the high expense and complexity brought by two complete injection systems renders it commercially challenged.
The present invention is directed to one or more of the problems set forth above.